1. Field of the Invention
The present invention relates to a Viterbi decoder and, more particularly, to means for achieving synchronization in a Viterbi decoder.
2. Description of the Prior Art
A Viterbi decoder is a decoder which uses the Viterbi algorithm to decode convolutional coded data and is known. Since the coding gain of the Viterbi decoder is high, it has been proposed to apply the Viterbi decoder technique to the field of communication, for cases in which the transmitting electric power is severely restricted or the quality of the transmission path is bad. Several techniques concerned with the decoder using the Viterbi algorithm are disclosed in the following U.S. Patents. In particular, the synchronizing technique in such a decoder is disclosed in detail in U.S. Pat. No. 3,872,432. The soft decision technique is described in U.S. Pat. No. 4,240,156 or 4,493,082. The multi-rate branch metric processor using the soft decision technique is also disclosed in U.S. Pat. No. 4,500,994.
When the Viterbi decoder is applied to a transmission path having a fairly low S/N ratio, it is necessary to obtain code synchronizing, between the transmission side and the reception side, so that the Viterbi decoding can be performed. In particular, when no frame sync signal is used, the synchronization must be obtained by the decoder itself on the basis of the reception data.
For this purpose, several methods of discriminating between synchronization and asynchronization are proposed in, for example, "Reports of Technical Researches (CS 82-43), The Institute of Electronics and Communication Engineers of Japan". Among these methods, the method which concerns use of the time-dependent change in state metric is simple in terms of hardware.
However, the conventional method is impractical because it has the drawback that when the S/N ratio of the transmission path is not specified, it is difficult to determine the threshold value which serves as the reference which is required for discriminating between synchronization and asynchronization.